Contact: David Sims
603-862-5369
Science Writer
Institute for the Study of Earth, Oceans, and Space

March 25, 2004

DURHAM, N.H. -- While climate may be impacted by carbon dioxide
emissions, aerosols, and other factors, a new study co-authored
by University of New Hampshire scientist George Hurtt offers further
evidence that land surface changes may also play a significant role.

Using data and computer models from NASA, the National Oceanic
and Atmospheric Administration and other agencies, the study of
summer climate in the United States reported that changes in land
cover, particularly vegetation, over the past 300 years may have
impacted regional temperatures and precipitation.

“Independent studies have suggested that the U.S. has warmed
on average since the Industrial Revolution. Our research suggests
that this warming might have been even greater had certain land
cover changes not occurred, “ said Hurtt of UNH's Institute
for the Study of Earth, Oceans, and Space and the Department of
Natural Resources. Somnath Baidya Roy, a research scientist at Princeton
University, was lead author of the study that appeared in a recent
issue of the Journal of Geophysical Research-Atmospheres. Co-authors
include Hurtt, Christopher Weaver of Rutgers University, and Stephen
Pacala, also of Princeton.

The study found that since 1700, land cover changes produced a
significant cooling effect of more than one degree Fahrenheit in
parts of the Great Plains and Midwest as agriculture expanded and
replaced grasslands. Farmlands tend to create lower temperatures
through increased evaporation. A warming effect was found along
the Atlantic coast where croplands replaced forests.

Compared to forests, croplands are less efficient in transpiration,
a daytime process where water evaporates from leaves during photosynthesis
and cools the air. A slight warming effect was also observed across
the Southwest, where woodlands replaced some deserts.

The study also found that land cover changes could impact local
precipitation, but not as significantly as they affect temperature,
because U.S. summer rainfall is not highly dependent on local land
cover and evapotranspiration. The relatively strong cooling over
the central U.S. has probably weakened the temperature difference
between land and the Gulf of Mexico, slowing the northward movement
of weather systems and resulting in enhanced rainfall across Texas.
Consequently, the air masses reaching the Central Lowlands region,
including Illinois and Indiana are drier, causing rainfall reductions.

“Land cover change is not uniform. Most people associate
land cover change with deforestation, but the changes in the U.S.
are more complex, creating a temperature signal that is more difficult
to study,” said Roy. The forest cover in the U.S. has actually
increased in the last 100 years mainly due to farm abandonment in
the East and fire suppression in the West. In addition, large parts
of the Great Plains have been converted into irrigated croplands,
which tends to produce cooling.

The research also carries additional implications. “It is
important to understand the effects of changing land cover, because
it can mitigate or exacerbate greenhouse warming,” said Roy.
“In the U.S. over the past 100 years, it seems to be offsetting
greenhouse warming. The opposite is probably true in most other
parts of the world. This finding has also been supported in previous
research.”

Unlike previous studies that simulated and compared past and present
climates with potential and current vegetation respectively, this
research used the Ecosystem Demography model in conjunction with
a regional climate model to track changes in land cover characteristics
for nearly 300 years. “The ED model is a technological breakthrough
and enables scientists to study the potential impacts of land use
and climate change across a wide range of scales, from individual
plants to continental regions,” said Hurtt.